A plasma containing ionized gases can be created by accelerating randomly occurring free electrons in an electric field until they attain sufficient energy to cause ionization of some of the gas molecules. Electrons formed in this ionization are in turn accelerated and produce further ionization. This progressive effect causes extensive breakdown of the gas accompanied by a rising level of electric current, and establishment of a discharge. This condition is often referred to as a discharge plasma. When sufficient energy has been applied, a steady state may be attained. At steady state there is an equilibrium between the rate of ion formation and the rate of recombination of the ions.
The electrical conductivity associated with discharge plasmas is caused by the drift of electrons in the electric field. Protons are also present in the plasma, but do not have a significant effect on the electric field because of their low drift velocity.
In addition to ionization, radical formation also occurs in a discharge plasma containing molecules consisting of two or more atoms. Radical formation is most often caused by the removal of one or more atoms from a molecule.
Plasma chemistry is the study of reactions of the species found in plasmas, i.e., atoms, free radicals, ions and electrons. The principles of plasma chemistry have been applied in such diverse areas as: chemical vapor deposition; substrate oxidation and anodization (such as formation of magnetic recording tape); and high temperature, high energy, plasma conversion of methane to acetylene (e.g. the Dupont arc acetylene process).
High energy hydrocarbon feedstocks such as ethylene and acetylene are vital to the petrochemical industry. However, these feedstocks are not found naturally in great abundance. One of the most prevalent hydrocarbon sources is natural gas. Natural gas contains over 90% methane, thermodynamically the most stable hydrocarbon. The energy needed to break one of the four C-H bonds of methane is about 415 kJ/mol.
Conversion of methane to other hydrocarbons to provide useful feedstocks is desirable, yet difficult due to the highly endothermic nature of the requisite conversion reaction. Typically, such conversion reactions have relied on high temperature reaction conditions. However, high temperature reactions are hard to control, and under such conditions it is difficult to prevent formation of unwanted by-products.
Industrial scale hydrocarbon cracking processes using plasma technology require extensive amounts of power in the form of electricity. For example, the Dupont acetylene process mentioned above, uses a plasma jet with a temperature of over 4000 K. This high temperature plasma jet is created by passing an electric current through a gaseous medium. The large amounts of electricity needed to create a high temperature plasma jet, and the poor selectivity (i.e., controllability) of the reaction and reaction products using such high temperature processes provide an incentive for the development of lower temperature reactions.
Other thermal techniques that have been employed to "crack" methane to form useful feedstocks include low and high frequency electrode and electrodeless discharge, triboelectric discharge, and laser irradiation. However, there are problems associated with each of these techniques, which make them unsuitable or impractical for large scale application. Electrical discharge results in coating of reactant on the electrode; triboelectric discharge involves potentially dangerous pressure changes, and is difficult to scale up. Laser irradiation is expensive and potentially corrosive to the reaction chamber.
Another technique which has been used in the search for an efficient cracking process for methane is microwave discharge. Microwave plasmas are created in the same manner as high temperature plasmas, although different microwave frequencies and less electric power is required to establish a plasma.
Several investigators have explored the use of plasmas in chemical reactions. McCarthy, J. Chem. Phys., 22:1360 (1954), obtained an energy yield of approximately 3600 kJ for each mole of C.sub.2 hydrocarbon produced using microwave discharge. McCarthy employed a pulsed microwave source at an output power level of 1500 watts.
One example of a relatively high efficiency reaction, not involving a plasma, is described in U.S. Pat. No. 4,574,038 to Wan, issued Mar. 4, 1986. Wan discloses a microwave-induced catalytic hydrocracking process for the selective conversion of methane to ethylene and hydrogen.
The method disclosed by Wan involves exposing methane and a microwave-absorbing catalyst to microwave energy, with pulsed microwave energy sufficient to convert the methane to ethylene and hydrogen. According to Wan, in order for the reaction to proceed with viable speed and selectivity, it is important that the catalyst be capable of attaining temperatures of 1400.degree. to 1600.degree. F.
In one example, Wan placed a Ni-Fe (85-15%) powder catalyst (0.1 g) in a reaction cell. The catalyst was pretreated with a stream of hydrogen and high power microwave radiation to remove oxide from the metal powder surface. Methane was then introduced to the reaction cell at a pressure of one atmosphere of methane. Wan applied a microwave energy source of 2.4 GHz at 100 watt incident power level to the gas stream. The microwave generator was operated to provide 5 second "on-time" pulses for a cumulative duration of 20 seconds irradiation with off-time rests of 20-60 seconds. By this technique, Wan obtained yields of 51.3% ethylene, 26.7% hydrogen and 21.8% methane. With other catalysts Wan obtained ethylene at 16% yield (Ni catalyst) and 14.6% (Co catalyst).
A major disadvantage of the Wan process, and other high power cracking processes, is that a heavy coke residue is deposited on the walls of the reactor and/or on the catalyst that is employed to accelerate the reaction. To maintain the reactor in operation the microwave induced reaction must frequently be discontinued and the residue removed. Hence, the reactor is frequently out of service. In Wan for example, the reactor is scrubbed with hydrogen gas to remove oxides which have contaminated the catalyst. In addition, the Wan process does not use a plasma, and the process entails pulsing the microwave power on and off. As a result, the Wan process is relatively inefficient. The catalyst must be scrubbed periodically, requiring a hydrogen stream and additional energy. In addition, the cracking reaction is stopped while the catalyst is scrubbed. Therefore, the Wan method does not offer continuous production of a desired reaction product.
By virtue of its widespread availability and low cost, methane is a desirable raw material for use in producing high energy hydrocarbon feedstocks. In addition to simple high energy hydrocarbon feedstocks such as ethylene, acetylene, propane, propylene, butane and butene, it is also desirable to produce oxygenated hydrocarbon feedstocks such as formaldehyde and methanol from methane. Thermal, non-plasma techniques can be used to oxidize methane at high temperatures (e.g., 300.degree.-700.degree. C.). However, this technique affords relatively low selectivity in terms of creating chemical bonds, and rupturing existing bonds in the raw starting material. Various catalysts such as metal oxides, non-metal oxides and mixed oxides have been used in these reactions. These catalysts include: MgO, Li-doped MgO, La.sub.2 O.sub.3, and mixtures of NaCl and MnO.sub.2. The yields observed with these catalysts range from about 0.1% to 30%.
It has been shown that discharge plasma processes involving methane gas as a reactant can produce radicals of H, CH.sub.3, CH.sub.2, and CH in the gas phase. When oxygen alone is used as the reactant, several radical species are obtained, including O, O.sub.2.sup.+ and others. Previous attempts to create a plasma from a mixture of hydrocarbons and oxygen using a glow discharge arrangement, resulted in the formation of completely oxidized hydrocarbon, i.e, CO.sub.2. Water and polymer deposits are also formed on the walls of the reactor. Nonetheless, oxygen-rich plasmas have been used commercially in adhesion processes and for selectively activating aromatic species.
Although microwave radiation has been used to crack methane, large quantities of power have conventionally been required to accomplish this objective, and substantial heat is evolved during the cracking process. Thus, the cost of electricity used to create the microwave radiation is a major factor in the low cost efficiency of feedstocks produced according to conventional microwave radiation plasma methods. In addition, the use of high power microwave radiation can rapidly foul catalysts used in the cracking process, resulting in additional loss of efficiency.